1. Field of the Invention
The present invention relates generally to ignition systems, and, more particularly, to a system for determining a fault mode in an ignition system and having an improved capacitive sensor apparatus.
2. Description of the Related Art
There has been much investigation into systems for determining absolute engine position information in an internal combustion engine. One known approach involves the use of a so-called compression sense system as seen by reference to U.S. Pat. No. 5,410,253 to Evans et al. As background, it is generally understood by those skilled in the art of ignition control that a relationship of proportionality exists between cylinder pressure magnitude and the magnitude of a breakdown voltage across a given spark plug gap. For example, in a direct ignition system (DIS), the spark plug in a cylinder undergoing compression requires a higher voltage across its gap for breakdown than does its counterpart spark plug in a cylinder undergoing a lower pressure exhaust event. Inasmuch as two spark plugs share a common source of ignition energy in such a direct ignition system, the spark plug in the high pressure cylinder will generally require more time to reach its breakdown voltage than will the plug in the lower pressure cylinder. This time difference is generally measurable. Evans et al. discloses a system that analyzes the time relationship of the discharge ignition voltage across pairs of spark plugs in such systems to provide direct information on which plug, and thus which cylinder, is in its compression stroke (or alternatively in its exhaust stroke). Absolute engine position information is needed to synchronize relative position inputs to an engine controller to provide for proper fuel delivery timing during the engine cycle. While known implementations of compression sense technology have eliminated the need for additional hardware (e.g., camshaft position sensor) to sense absolute engine position, certain fault modes in the engine and/or ignition system have, heretofore, prevented full utilization of compression sense system outputs.
Concerning particular compression sense implementations, it is further known to use a capacitive sensor to sense the breakdown events, and thus the relative time differences, as described above as seen by reference to U.S. Pat. No. 5,561,379 to Downey. Downey discloses a pair of planar conductive plates remote from each of the leads of a secondary winding of an ignition coil to capacitively couple ignition voltages to a common node. The common node is coupled as an input to a processing circuit for determination of absolute engine position. Such secondary winding leads, and planar conductive plates are each at least partially immersed in an epoxy potting material, which forms a dielectric for capacitive coupling therebetween. Downey discloses an air space between the exposed surfaces of the potting material in which the conductive plates and the winding leads are immersed. That is, Downey discloses three, stacked layers of dielectric material between the xe2x80x9cplatesxe2x80x9d of the sensing capacitor: (i) a first potting material layer; (ii) an xe2x80x9cairxe2x80x9d layer; and, (iii) a second potting material layer. The dielectric contribution of the xe2x80x9cairxe2x80x9d layer, however, varies based on changing conditions (e.g., introduction of water into such air layer), thereby presenting challenges to the designers of circuitry for processing the sensed ignition voltages.
Accordingly, it would be desirable to provide an ignition system, including a suitable sensing structure, that improves on the known systems described above.
The present invention provides accurate information regarding engine absolute position, even when engine or ignition system fault modes are present. In addition, an improved, integral sensing element is configured to capacitively sense spark discharges associated with multiple ignition coils, and further, is configured so that an effective capacitive dielectric constant is maintained relatively constant.
In one aspect of the present invention, a method for determining absolute engine position is provided. The method is suitable for use in an ignition system for a multi-cylinder internal combustion engine. There are four basic steps. The first step involves defining a cylinder identification signal indicative of a respective combustion event in each of the cylinders. The next step involves providing a data structure. The data structure includes an input parameter and an output parameter. The input parameter has a plurality of values corresponding to the cylinder identification signal in the presence of one or more fault modes. In a preferred embodiment, the input parameter may comprise a 4-bit CAM code, which is the cylinder identification signal sampled four times at predetermined intervals during one complete firing sequence of the engine. Further, each input parameter has a respective output parameter associated therewith indicative of absolute engine position. In one embodiment, the output parameter may comprise a value indicative of which cylinder was last under compression. The third step involves generating the cylinder identification signal in accordance with a compression sense detection strategy (i.e., during operation). Finally, the last step involves selecting one of the output parameters contained in the data structure using the generated cylinder identification signal. Advantageously, the method provides absolute engine position, even during occurrence of fault modes.
In another aspect of the present invention, a method of determining a fault mode in an ignition system is provided. The method is suitable for use in an internal combustion engine having a plurality of cylinders. The method includes the step of defining a plurality of fault modes associated with the engine or the ignition system as a function of a cylinder identification signal. The cylinder identification signal is indicative of an occurrence of a combustion event in each of the respective cylinders. The next step involves generating the cylinder identification signal in accordance with a compression sense detection strategy. Finally, the last step involves selecting at least one of the fault modes using the cylinder identification signal. This information may be provided to service technicians for improved servicing.
In yet another aspect of the present invention, a direct ignition apparatus is provided which includes a housing, a pair of ignition coils, a sensing conductive element, dielectric material, and an ignition signal processing circuit. The pair of ignition coils are disposed in the housing, and each coil has a secondary winding configured to develop an ignition voltage at respective first ends thereof. Each one of the first ends of the secondary windings is configured to be connected to first and second spark plugs. Each plug is disposed in a corresponding cylinder of the internal combustion engine. The ignition voltage so developed is configured to cause the spark plugs to produce a respective spark discharge. The sensing conductive element may include a generally planar portion disposed a predetermined distance from the pair of ignition coils. Preferably, the generally planar portion may be located proximate the leads of the secondary windings for increased capacitive coupling.
Advantageously, the dielectric material substantially occupies the space between the ignition coils and the planar portion of the sensing conductive element. That is, there is no xe2x80x9cairxe2x80x9d layer that may be subjected to changing conditions that would change the capacitive dielectric constant. During spark discharge, the flow of spark current is capacitively coupled to the sensing conductive element to produce a corresponding ignition signal voltage. The ignition signal processing circuit is electrically connected to the sensing conductive element, and is configured to generate, in a preferred embodiment, a cylinder identification signal indicative of an occurrence of an ignition event.
Other objects, features, and advantages of the present invention will become apparent to one skilled in the art from the following detailed description and accompanying drawings illustrating features of this invention by way of example, but not by way of limitation.